1. Field of the Invention
The present invention relates in general to improvements in the construction of heat exchangers of the tube bundle type. The improvement is for protecting the heat transfer tubes from damage by contact with solid debris inadvertently contained within the working fluid which flows through the tube bundle to effect heat transfer therebetween.
2. Description of the Prior Art
Heat exchangers of the tube bundle type have been employed in a wide variety of applications to effect heat transfer to or from the working fluid. One such application is the use of a heat exchanger commonly referred to as a steam generator for converting the working fluid, i.e., water, into steam for the generation of electrical power. Such steam generators are usually constructed as a bundle of tubes extending from a supporting tubesheet and contained within a cylindrical wrapper. The cylindrical wrapper defines an annular downcomer passage between itself and the concentric outer shell. A mixture of fresh feedwater plus return water from a liquid/vapor separator enters at the tube bundle base through an opening in the wrapper at the bottom of the downcomer passage at the tubesheet. This opening, being arranged at the lowest elevation, is a natural collection point for solid debris left behind during steam generator manufacture or created by later repairs, internal breakage and/or wear during normal operation. Water, at this opening for entry into the tube bundle, is in a highly turbulent state because of sharp changes in the downcomer passage flow geometry and the 90.degree. change in direction it must follow. Any debris at the bottom of the downcomer passage will be picked up by the vigorous water flow and thrown against the exposed thin-walled heat transfer tubes within the tube bundle. The impact of the solid debris with the heat transfer tubes can often cause tube rupture. Because of this turbulence and the flow changes caused by electrical load variations, the solid debris will not remain at the same location within the tube bundle. Such debris usually moves about the lower portion of the tube bundle, rubbing and wearing against the tubes, and is often thrown against those heat transfer tubes. Typical debris found in steam generators at this location can vary in size from one-half inch hexagonal nuts to pieces of steel broken loose from weldments serving as wrapper to shell supports.
Accordingly, it can be appreciated that there is an unsolved need to provide a protection apparatus for preventing impact and wear damage to the thin-walled heat transfer tubes within a tube bundle of a heat exchanger which is subject to repeated contact with solid debris contained within the working fluid.